Sensor devices have been proposed for determining the location of a pressure signal as produced, e.g., by a stylus or by digital touching; such a device may take the form of a writing pad or of a touch screen for entering data into a computer system.
Some devices of this type utilize two mutually orthogonal electrical fields and a movable probe to obtain electrical signals corresponding to x-y coordinates of the position of the probe; others operate with an inactive pointer, such as, e.g., a finger or stylus, by measuring echo signal transit times, light beam interruptions, capacitance change of distributed tabs, or forces at the periphery of the surface. The following documents are cited as representative of the state of the art.
U.S. Pat. No. 4,071,689, issued Jan. 31, 1978 to J. E. Talmadge et al. discloses an electrographic sensor comprising a rigid, optically transparent substrate which supports a uniformly resistive layer, and a resistor network around the perimeter of the substrate. The device operates with a conductive stylus and an oscillator.
U.S. Pat. No. 4,121,049, issued Oct. 17, 1978 to F. R. Roeber discloses devices which rely on z-direction mechanical displacement transverse to the x-y plane of an input surface. Springs, differential transformers, or variable-reluctance pickups are used.
U.S. Pat. No. 4,214,122, issued July 22, 1980 to V. B. Kley discloses a resistive planar graphical input insulating substrate. Conductor strips around the periphery of the resistive layer are designed to enhance linearity of device performance.
U.S. Pat. No. 4,293,734, issued Oct. 6, 1981 to to W. Pepper, Jr. discloses a method for determining the position of a source or sink of electric current on the surface of a resistive element or impedance layer.
U.S. patent application Ser. No. 682,780, filed Dec. 18, 1984 by J. Federico et al. discloses the production of position-related electrical signals upon local contact between two conductive surfaces. The two surfaces may be spaced apart by various means such as, e.g., by a layer of a fluid insulating medium, by movable microspheres, by rubber pedestals which may be impregnated with conductive particles, or by a photoconductive layer.
For a general review of the field of location sensors see, e.g., the paper by D. J. Grover, "Graphics Tablets--A Review", Displays, Volume 1 (1979), pp. 83-93.
Also of interest in connection with the invention is U.S. patent application Ser. No. 728,813, filed Apr. 30, 1985 by S. Jin et al.